Therefore when they crossed this zone at 33,000 feet, they were neither cutting it razor-close nor bending the rules, but doing what many other airlines had done, in a way they assumed was both legal and safe. Legal in much the way that driving 63 in a 65-mile-per-hour zone would be.

And safe, not just for regulatory reasons, but because aircraft at cruising altitude are beyond the reach of anything except strictly military antiaircraft equipment. During takeoff and landing, airliners are highly vulnerable: They are big, they are moving slowly and in a straight line, they are close to the ground. But while cruising, they are beyond most earthbound criminal or terrorist threats.

This is why, even during wartime, airliners have frequently flown across Iraq and Afghanistan. The restricted zone over Ukraine was meant to protect against accidental fire or collateral damage. It didn’t envision a military attack.